Method for coded modulation taking account of the error sensitivity of the user data and encrypting said data after coding

ABSTRACT

A method of coded modulation of digital data is described, the digital data being divided into parallel signal streams, and the useful bits then being channel coded at different code rates according to their error susceptibility. The useful bits channel coded differently in this way are then scrambled separately in time. The different code rates used and the respective number per transmission frame of the useful bits channel coded at different code rates are signaled to the receivers, so that the receiver is able to perform the descrambling and channel decoding.

BACKGROUND INFORMATION

[0001] The present invention is based on a method of coded modulationaccording to the preamble of the independent claim.

[0002] It is already known that coded modulation may be used, jointlyoptimizing the channel coding and modulation. The term multilevel codingis equivalent to coded modulation. Source coding for data reduction isperformed before coded modulation.

ADVANTAGES OF THE INVENTION

[0003] The method according to the present invention for codedmodulation having the features of the independent claim has theadvantage over the related art that a non-uniform error protection isused in coded modulation, using different code rates for useful bitshaving a different error susceptibility. The code rate and number ofuseful bits to be coded may be selected independently of one another.

[0004] The method according to the present invention permits a simplerimplementation on the whole.

[0005] Advantageous improvements on the method of coded modulationcharacterized in the independent patent claim are possible through themeasures characterized in the dependent claims.

[0006] It is especially advantageous that the useful bits, each channelcoded at different code rates, are distributed among parallel scramblersfor scrambling chronologically and are subsequently combined back intoone signal stream by a multiplexer. Thus advantageously only the usefulbits that have been channel coded at the same code rate are alsoscrambled together. The channel decoding and descrambling which precedechannel decoding in the receiver are thus simple accordingly.

[0007] As an alternative, it is advantageous that an scrambler is usedper step downstream from the coder to scramble the useful bits channelcoded at different code rates separately and in successionchronologically. Thus, an scrambler is used which has the necessaryintelligence to divide the useful bits channel coded at different coderates into corresponding groups and then scramble them chronologically.

[0008] It is also advantageous that the at least two different coderates used for channel coding and the respective number of useful bitschannel coded at different code rates are signaled to the receiver.Thus, the receiver is able to determine the code rates at which theuseful bits having different error susceptibilities have been channelcoded and the number of differently coded useful bits for implementing acorresponding descrambling and channel coding correctly.

[0009] It is also advantageous that both a transmitter and a receiverhave means for implementing the method according to the presentinvention.

DRAWING

[0010] Exemplary embodiments of the present invention are illustrated inthe drawing and explained in greater detail in the followingdescription.

[0011]FIG. 1 illustrates partitioning of 4-ASK;

[0012]FIG. 2 shows a block diagram of the transmitter according to thepresent invention;

[0013]FIG. 3 shows a block diagram of the receiver according to thepresent invention; and

[0014]FIG. 4 shows a flow chart of the method according to the presentinvention.

DESCRIPTION

[0015] The Digital Radio Mondiale (DRM) digital transmission system fortransmission bands of less than 30 MHz is currently being developed. Ithas been decided that a multilevel coding (MLC) is to be used as thechannel coding. The channel coding and modulation are optimized jointly,which is why this is also known as coded modulation. Channel coding addsredundancy to data so that transmission errors may be detected andoptionally corrected.

[0016] In a higher-level modulation method having a q-nary signalconstellation, the signal alphabet has exactly q values. The basis forMLC is the partitioning of the signal alphabet into subsets. Eachdivision step is assigned one component of the address vector of thesignal space representation. Each component is protected with its owncode here. If we consider a 2^(m)-step signal constellation, this yieldsa division into n steps, if m=n, according to address vector c (=c₀, c₁,. . . , c_(n-1)). Steps m of modulation need not necessarily be equal tothe number of steps if using 64-QAM (quadrature amplitude modulation).

[0017]FIG. 1 illustrates partitioning of 4-ASK (amplitude shift keying).Four states are thus coded in 4-ASK. The data stream is coded with nparallel coders, and code C₀ has the lowest code rate R₀, i.e., it addsthe greatest redundancy and protects the position of the address vectormost susceptible to errors. Four states represented by solid circles areshown on the top state bar in FIG. 1. Individual codable states arearrived at in 4-ASK via the two middle state bars. The first stepbecomes either c₀=0 or c₀=1. Accordingly, the four solid circles aredistributed among two numerical bars having complementary solid andempty circles. In the four bottom state bars, the individual states in4-ASK are coded, namely 00, 01, 10 and 11. State 00 has one solid circleat the far left, followed by three empty circles. State 01 has a solidcircle in third place from the left. State 10 has the solid circle insecond place from the left, and state 11 has the solid circle at the farright. The other positions are represented by empty circles for a 0.

[0018]FIG. 2 shows a block diagram of the transmitter according to thepresent invention. Data to be sent using the transmitter according tothe present invention is stored in data memory 1. However, other datasources may also be used here. This data is then transmitted from datamemory 1 to a source coder 2 which performs source coding to reduce theamount of data to be transmitted. The data thus source coded togetherwith the useful bits is then transmitted to a bit multiplexer 3 whichdistributes the data stream among n parallel lines. A coder whichchannel codes one of the data streams (q₀ . . . q_(n-1)) is connected toeach of these n lines numbered continuously 0 through n−1. For example,a coder 5 is shown here in line 0, and a coder 4 in line n-1 . Signalsc₀ and c_(n-1) are available here at the output of the respective coder.Coders 4 and 5 perform channel coding by using a convolution coding andthus add redundancy back to the useful bits. Coders 4 and 5 use twodifferent code rates here. Source coder 2 has identified the useful bitswhich require a first error protection and the remaining useful bitswhich require a second error protection. To do so, channel coders 4 and5 then each use a different code rate. The useful bits are thusdistributed among the steps according to specified rules without ananalysis of the useful bits in this regard. First the useful bits aredistributed among the steps and coders 4 and 5 which are channel codedwith the first code rate, and then these are followed by useful bitschannel coded at the second code rate. To do so, the code rate on allavailable coders is switched.

[0019] Since differently channel-coded useful bits are now available atthe output of coders 4 and 5, they are scrambled chronologicallyseparately in downstream scramblers. A multiplexer 61 downstream fromcoder 5 and likewise a multiplexer 51 downstream from coder 4 distributethe differently coded useful bits among scramblers 62 and 63 as well as52 and 53. The term scrambler originates from English. Scrambling meansthat bits which are close together in time and contain informationhaving a high correlation with adjacent bits are separated from oneanother in time to allow correction of as many errors as possible on thereceiving end in the case of a burst error having channel decoding tominimize the number of audible errors. Scrambled useful bits are thencombined again into one signal stream in demultiplexer 54, starting fromscramblers 52 and 53. Scrambled useful bits of scramblers 62 and 63 arecombined again to form one signal stream in demultiplexer 64 by analogy.

[0020] Data thus channel coded is then assigned to signal space pointsin block 6 to then generate the respective modulation symbols.

[0021] Convolution codes having punctuation are used as the componentcodes in individual coders 4 and 5. In MLC, at least two different coderates are used for each step here. After modulation in function block 6,signaling data is added to the signal stream, signaling to a receiverwhich different code rates have been used for channel coding and howmany bits per frame have been channel coded at the first code rate andhow many bits have been channel coded at the second code rate. It isthen possible for a receiver to perform correct demodulation anddescrambling as well as channel decoding.

[0022] Then in FIG. 2 the modulation symbols thus coded are transmittedfrom function block 6 to OFDM (orthogonal frequency division multiplex)modulator 7, which distributes the individual modulation symbols tofrequency carriers which are situated close together and are mutuallyorthogonal. The resulting OFDM signals are then mixed in an analog RFpart 8, amplified and then sent out using an antenna 9.

[0023]FIG. 3 shows a block diagram of the receiver according to thepresent invention. An antenna 10 for receiving the OFDM signals isconnected to an input of an RF receiving part 11. RF receiving part 11converts the received signals into an intermediate frequency, amplifiesthem and filters them. In addition, RF receiving part 11 transmits thesesignals to a digital part 12 which digitizes the received signals andperforms OFDM demodulation. The modulation symbols obtained in this wayare then demodulated in processor 13, subjected to descrambling, channeldecoding and source decoding to convert them to analog signals. Theanalog signals are then amplified by an audio amplifier 14 to ultimatelybe played back via loudspeaker 15. In particular the signaling dataindicating the two code rates used and the number of useful bits perframe for a code rate is used after analysis. It is then possible for areceiver to perform a correct descrambling and channel decoding.

[0024] Instead of two code rates, it is also possible to use multiplecode rates. Instead of source coder 2, bit multiplexer 3 may also decidewhich bits are to be channel coded at which code rate. This may also beassigned as a task to a higher-level processor which is provided in thetransmitter according to the present invention. As an alternative, it isalso possible here to receive multimedia data which is then played backvisually.

[0025]FIG. 4 illustrates the method according to the present inventionfor coded modulation in the form of a flow chart. In method step 16,data is made available by data memory 1 and subjected to source codingby source coder 2. In addition, source coder 2 here determines whichuseful bits are channel coded at which code rate. Non-uniform errorprotection is thus established. In method step 17 the resulting datastream is divided into parallel data streams by bit multiplexer 3. As analternative, it is also possible here for bit multiplexer 3 to performthe division of useful bits for the different channel coding. In methodstep 18, individual coders 4 and 5 perform the channel coding. Theuseful bits are channel coded at two different code rates, as defined bysource coder 2. Useful bits, which are thus channel coded differently,are scrambled chronologically in method step 19. Only useful bits whichhave been channel coded at the same code rate are scrambledchronologically. This may be accomplished either by parallel-connectedscramblers or by an intelligent scrambler which sequentially scramblesuseful bits channel coded at different code rates separately from oneanother. In method step 20, the channel-coded useful bits thus formedare assigned to signal space points to generate the modulation symbolsin function block 6. In method step 21 the modulation symbols aresubjected to OFDM modulation, and in method step 22 the OFDM signals areamplified and transmitted. In addition, the two different code rates forthe useful bits and the number of useful bits per code rate used pertransmission frame are also transmitted as signaling from thetransmitter to the receive, so the receivers are capable of descramblingand channel decoding the differently channel-coded useful bits.

What is claimed is:
 1. A method of coded modulation of digital data, the digital data having useful bits; and the coded modulation being implemented in multiple steps; the useful bits being classified according to their error susceptibility; the useful bits are divided into parallel signal streams; each signal stream is channel coded at at least two different code rates per step by a coder (4, 5); the different code rates being used as a function of the error susceptibility of the useful bits; useful bits channel coded at different code rates are scrambled separately chronologically; and the channel-coded and scrambled useful bits are then assigned to signal space points to generate modulation symbols.
 2. The method according to claim 1, wherein useful bits channel coded at different code rates are distributed among parallel scramblers (52, 53, 62, 63) per step for scrambling chronologically and are subsequently combined back into one signal stream by a demultiplexer (54, 64).
 3. The method according to claim 1, wherein an scrambler scrambles the useful bits channel coded at different code rates separately and in succession chronologically per step downstream from the coder (4, 5).
 4. The method according to one of the preceding claims, wherein the at least two different code rates and a respective number of useful bits channel coded at different code rates are signaled to a receiver.
 5. A transmitter for implementing the method according to one of claims 1 through 4, wherein the transmitter has a first multiplexer (3), a coder (4, 5) for channel coding per step, an scrambling unit (51, 52, 53, 54, 61, 62, 63, 64) per step and means for assigning the channel-coded useful bits to the signal space points (6).
 6. The transmitter according to claim 5, wherein the transmitter has an scrambling unit having an scrambler.
 7. The transmitter according to claim 5, wherein the transmitter has, per scrambling unit, a second multiplexer (51, 61), a demultiplexer (54, 64) and an scrambler per code rate (52, 53, 62, 63) used.
 8. A receiver for implementing the method according to one of claims 1 through 4, wherein the receiver has means for analyzing the signaling (13) for demodulation of the modulation symbols, for channel decoding and for unscrambling. 